Taking the outside view means using an estimate based on a class of roughly similar previous cases, rather than trying to visualize the details of a process. For example, estimating the completion time of a programming project based on how long similar projects have taken in the past, rather than by drawing up a graph of tasks and their expected completion times. The planning fallacy is that people tend to be hugely optimistic when visualizing the details of a case, and become even more optimistic as they visualize more details.
Example 1: Japanese students expected to finish their essays an average of 10 days before deadline. The average completion time was actually 1 day before deadline. When asked when they'd completed similar, previous tasks, the average reply was 1 day before deadline. (Buehler, R., Griffin, D., & Ross, M. 2002. Inside the planning fallacy: The causes and consequences of optimistic time predictions. Heuristics and biases: The psychology of intuitive judgment, 250-270. Cambridge, UK: Cambridge University Press.)
Example 2: Students instructed to visualize how, where, and when they would perform their Christmas shopping, expected to finish shopping more than a week before Christmas. A control group asked when they expected their Christmas shopping to be finished, expected it to be done 4 days before Christmas. Both groups finished 3 days before Christmas. (Buehler, R., Griffin, D. and Ross, M. 1995. It's about time: Optimistic predictions in work and love. European Review of Social Psychology, Volume 6, eds. W. Stroebe and M. Hewstone. Chichester: John Wiley & Sons.)...